Ramp travel index or RTI, is a way of measuring a vehicle's ability to flex its suspension, a property also known as axle articulation. The RTI rating is used mainly in the off-roading industry to test and describe chassis limits of modified vehicles. The method for measuring RTI is not standardized. However, normally, the ramps vary between 15 and 30 degrees of angle for the vehicle to ride up. “Ramping” a vehicle involves putting one front tire on the ramp and driving up slowly until one of the other three tires (usually the rear one on the same side as the tire driving the ramp) begins to leave the ground. The measurement is only taken when the other three tires are still on the ground. The distance traveled up the ramp is then measured and is divided by the vehicle's wheelbase and finally multiplied by 1000 to give a final RTI score. Most stock SUVs have RTI values from 400 to 550; vehicles modified for off-road competition have the ability to exceed 1000.
However, the RTI suffers from a number of drawbacks. For example, the ramp angle can vary which leads to inconsistencies (for example, the same vehicle can have an RTI of 1033 when measured on a 20-degree ramp, an RTI of 836 when measured on a 25-degree ramp and an RTI of 706 when measured on a 30 degree ramp). E.g., some vehicles may have a higher RTI compared to other vehicles at a low ramp angle but a lower RTI compared to the same vehicles at a high ramp angle. Further, there isn't a standard for certain vehicle characteristics, such as tire pressure, to ensure consistency across measurements. In addition, driver skill can influence the final RTI. Moreover, the location of where measurement occurs is not standardized. I.e., any one of closest point of contact to the bottom of the ramp, farthest point of contact to the bottom of the ramp, center point of contact to the bottom of the ramp, the point on the ramp matching the height of the axle and the lowest point of the tire on the ramp may all be measured and used to calculate the RTI. Finally, other factors, such as direction which the vehicle is driven (forward or reverse), approach angle and weight distribution all may change the value of the RTI. All of these inconsistencies mean that RTI is not an inherent characteristic of axle articulation.
In addition, measuring RTI is an inherently unsafe procedure. Obviously, a mistake in driving can lead to the vehicle coming of the ramp which has the potential to injure both the driver and bystanders. Further, the vehicle can move when the vehicle is put into or taken out of gear, when the brakes are engaged or when the vehicle is shut off. Each of these can lead to changes in measurement and/or safety hazards.
Accordingly, there is a need in the art for an axle articulation measurement that is free of inconsistencies (i.e., that gives repeatable results). Further, there is a need in the art for an axle articular measurement that is safe for both the driver and bystanders.